ci: add GitHub Actions check (#6)

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/nix.yml

@@ -0,0 +1,16 @@
+name: Nix
+on: [ push ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          # Nix Flakes doesn't work on shallow clones
+          fetch-depth: 0
+      - uses: cachix/install-nix-action@v10
+        with:
+          install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20200618_377345e/install
+      - run: echo "experimental-features = nix-command flakes" | sudo tee -a /etc/nix/nix.conf
+      - run: nix flake check